Electroless or autocatalytic coating of dielectric surfaces is a well known process finding widespread utility in the preparation of such diverse articles as printed circuits, automotive trim, mirrors, etc.
Normal commercial electroless coating processes generally involve an initial cleaning and etching of the dielectric substrate by physical or chemical means to improve adherence of the metallic coating. Etching also renders the substrate a greater hydrophilic nature. The etched surface is then catalyzed by suitable catalytic compositions and processes to provide a surface capable of electroless plating initiation. In the prior art, the catalytic treatment generally encompasses the use of precious metals. More recently, compositions and processes utilizing non-precious metals have been disclosed suitable for electroless plating of dielectrics. U.S. Pat. Nos. 3,958,048; 3,993,491; 3,993,799; 4,167,596; 4,136,216; 4,180,600; 4,151,311; 4,150,171; 4,048,354; 4,181,759; 4,087,586; 4,082,899; 4,131,699; 3,993,801; 3,993,848 and 4,132,832 show that colloidal systems based upon non-precious metals could constitute the basis for new commercial plating processes. These patents are included herein by reference. Also of interest are the following patents and article, noted in the way of prior art: Japanese Patent 49/126,999, U.S. 3,949,121, British Patent 1,426,462, and Weiser "Inorganic Colloid Chemistry", Vol. I, pp. 1-5 and pp. 137-143, John Wiley & Sons, Inc. (1933) which disclose the prior art as well as the recent advancements in which non-precious metals have been reported.
In reviewing some of the teachings disclosed, it is evident that colloids of either hydrous oxides, metals (elemental state) and alloys (phosphides, borides, nitrides, copper/tin, etc.) are useful in the catalytic treatment either as a two step or a single step activation treatment. Generally speaking, preferred metals in the above colloids are selected from the group consisting of cobalt, copper, iron and nickel although, as suggested in U.S. Pat. No. 3,993,799, other non-precious metals may be used. It is recognized that it is generally desirable to have suspensions (dispersions) of very fine particulate matter for both stability (i.e., against precipitation), reactivity, and adhesion to the substrate. Accordingly, it is highly desirable to prepare such suspensions under conditions which would yield finely divided and highly stable colloids with good adsorption.
It is also well recognized in the art of electroless plating that for effective electroless plating onto catalytically treated non-conductors, at least one of the following requirements must be met:
Case I: The catalytic surface may react chemically with the reducing agents present within the electroless plating bath. More than one chemical reaction may take place.
Case II: The catalytic surface may react chemically with the metallic ions present within the electroless plating bath in a galvanic type replacement reaction.
In Case I, the chemical reactions may range from chemical reduction of the catalytic components present on the dielectric, and/or decomposition of the reducing agent at the interface ultimately yielding hydrogen gas via an active reducing agent intermediate. In Case II, to permit a galvanic replacement reaction, it is recognized that some of the metal ions present in the solution must be more noble with respect to the metal and metal ions present on the treated non-conductor surface. Such relationship is well recognized from the EMF series. Thus, while metals like copper, cobalt, nickel and iron may be preferred as recognized in U.S. Pat. No. 3,993,799, yet other non-precious metals may also be of potential use (e.g., zinc, manganese, etc.). It is further recognized that it is highly desirable to have catalysts which when contacted with the chemical (electroless) plating bath will yield short induction times which generally takes place prior to the actual metal deposition. Generally speaking, it is recognized that whenever the induction time is short, the probability for complete metallic coverage is better and thus eliminates the problem of skip plating.